VERITAS Discovery of VHE Gamma Rays from the Starburst Galaxy M82 - - PowerPoint PPT Presentation

▶
veritas discovery of vhe gamma rays from the starburst
SMART_READER_LITE
LIVE PREVIEW

VERITAS Discovery of VHE Gamma Rays from the Starburst Galaxy M82 - - PowerPoint PPT Presentation

Feb 24, 2023 416 likes •564 views

VERITAS Discovery of VHE Gamma Rays from the Starburst Galaxy M82 Niklas Karlsson for the VERITAS collaboration Astronomy Dep., Adler Planetarium (Chicago) The 2009 Fermi Symposium - Washington, D.C. - 2-5 Nov 2009 VERITAS Very Energetic

slide-1
SLIDE 1

The 2009 Fermi Symposium - Washington, D.C. - 2-5 Nov 2009

VERITAS Discovery of VHE Gamma Rays from the Starburst Galaxy M82

Niklas Karlsson for the VERITAS collaboration Astronomy Dep., Adler Planetarium (Chicago)

slide-2
SLIDE 2

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 2

VERITAS

Very Energetic Radiation Imaging Telescope Array System

  • Mt. Hopkins, AZ
  • 1268 m a.s.l.
  • Four 12m telescopes
  • f/D~1.0
  • 350 mirrors; ~110m2
  • 499 pixel cameras
  • 3.5° FOV
  • 3-level trigger system
  • ~250 Hz rate

T2 T1 T3 T4 Moved T1 in summer 2009

  • Energy threshold ~150 GeV
  • Sensitivity 1% Crab (5σ) in < 50h
  • Angular resolution <0.1° (r68%)
  • Energy resolution ~15%

Currently the most sensitive array 30% improvement in integral flux sensitivity above 300 GeV

See Perkins et al. poster “VERITAS Telescope 1 Relocation”

slide-3
SLIDE 3

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 3

Origin of Cosmic Rays

  • Existence well established near Earth
  • First evidence in 1912 (Hess)
  • But the origin has eluded us for 100+ years!
  • Diffuse γ-rays from the Milky Way
  • Interpreted as mainly coming from CRs

interacting with interstellar gas

  • CRs + ISM → π0 → γ-rays
  • electrons + ambient photons → γ-rays
  • Where are these CRs accelerated?
  • Supernova remnants
  • Massive star winds
  • Can we look elsewhere for more

evidence?

  • LMC - nearby, observed with EGRET and

Fermi-LAT

  • Other galaxies

H.E.S.S. RX J1713

slide-4
SLIDE 4

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 4

Why Starburst Galaxies?

  • Starbursts activity induced by galaxy interactions/mergers
  • Strong tidal forces
  • Active star-forming regions
  • Leads to high gas densities & star formation rates
  • High supernova rate
  • Shocks from massive star winds and supernovas
  • Enhanced cosmic-ray flux ⇒ enhanced gamma-ray flux
  • Requirements for good candidates
  • Distance - nearby
  • High CR density
  • Measure via synchrotron emission in radio frequencies
  • High mean gas densities
  • Form far infrared (FIR) emission
  • Modeling
  • M82 (Persic et al. 2008)
  • NGC 253 (Domingo-Santamaria et al. 2005)
slide-5
SLIDE 5

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 5

M82 - prototypical starburst

  • Among the closest starbursts
  • Core starburst region
  • SF rate ~10x Milky Way
  • SN rate ~0.1/yr
  • CR density ~100x Milky Way
  • Inferred from synchrotron emission
  • Gas density ~150 cm-3
  • Weak upper limits from previous

generation observatories

  • EGRET (HE)
  • HEGRA & Whipple (VHE)
  • flux <10% Crab

NASA, ESA, The Hubble Heritage Team, (STScI / AURA) Chandra

slide-6
SLIDE 6

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 6

VERITAS Discovery

  • M82 observed 2007-2009
  • Quality selection (weather etc.)
  • ~137 h live time (deepest VERITAS

exposure to date)

  • Standard VERITAS analysis
  • Std. practice to use 3 sets of cuts
  • Theoretical prediction of a hard spectrum
  • Expect a hard cut to be the best
  • Cuts a priori optimized using Crab data at

θ ≈ 40°

  • Eth ≈ 700 GeV (lower sensitivity at

θ ≈ 40°)

  • Point-like excess of 91 γ ⇒ 5.0σ
  • 4.8σ post-trials significance
  • The results are now published in Nature
  • nline.

θ2 [deg] Events

slide-7
SLIDE 7

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 7

M82: Steady VHE γ-ray Source

  • One of the weakest VHE γ-ray

sources ever detected

  • 0.9% of the Crab Nebula

(E>700 GeV)

  • 0.6 γ/hour
  • Cumulative excess consistent

with a steady flux

  • Lightcurve is consistent with no

monthly variation

  • χ2=11.5 with 9 d.o.f.
  • P(χ2)=0.24
slide-8
SLIDE 8

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 8

VHE γ-ray Spectrum of M82

  • Energy range 875 GeV to 5 TeV
  • Power-law fit
  • dN/dE∝(E/TeV)-Γ
  • Γ = 2.6±0.6
  • Close to model predictions
  • Pohl (1994)
  • Völk et al. (1996)
  • Persic et al. (2008)
  • de Cea del Pozo et al.(2009)

VERITAS spectrum Model of Persic et al. (2008)

slide-9
SLIDE 9

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 9

Interpretation

Hadronic channel

  • CR ions + matter → π
  • π → γ and sec. electrons
  • Secondary electron emit

synchrotron radiation

  • Radio frequency 32 GHz
  • Constrain γ-ray flux from CRs at 20

GeV

  • Extrapolated VERITAS spectrum

gives ~2x too high flux

  • Γ = 2.3 ok though
  • Spectrum is harder at Fermi-LAT

energies OR VHE flux not predominantly from CR ions

Leptonic channel

  • Inverse Compton scattering
  • CR electrons + photons → X-rays

and γ rays

  • Use non-thermal X-ray emission to

constrain the electron population

  • Lower limit on magnetic field (8 nT)
  • Upper limit on absolute number of

electrons at about 1 GeV

  • But 10 TeV electrons required for

VHE gamma rays

  • Theory predicts Γ = 2.0 in the 100

keV to 100 GeV energy band

  • Steepening of IC spectrum and a

cut off at some energy due to cooling

slide-10
SLIDE 10

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 10

Summary

  • VERITAS has discovered VHE γ-ray emission from M82
  • 91 γ’s in 137 h of quality-selected live time
  • Post-trial significance is 4.8σ
  • Steady flux F(E>700 GeV)=(3.7±0.8stat±0.7syst) ×10-13 cm-2s-1
  • Luminosity is ~2 ×1032 W; approx. 0.03% of the optical luminosity
  • Weakest VERITAS source to date
  • First clear detection of VHE gamma rays from an

extragalactic object of non-AGN type

  • Hard spectrum source
  • Γ = 2.6±0.6
slide-11
SLIDE 11

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 11

Systematics Checks

  • All hardware operating normally, no moonlight data & dark NSB region
  • “Hard cuts”: Enormous images (>200 PE); bright star effects mitigated; very low

background (S/N ~ 1/3)

  • Result verified (5.2σ) by independent analysis/calibration/simulation package(s)
  • Alternate background estimation: Ring method => 5.1σ on-source
  • Also ~5σ using a binned maximum-likelihood method
  • Reflected-region BG method always has 11 off-source regions
  • Significance distribution is Gaussian (mean 0, σ = 1)
  • No bias in long data set: Stack extragalactic non-blazar data
  • With the same analysis: Combined excess of -4 events (-0.2σ) in ~121 h of live-time (no

moonlight data)

  • Not due to brightness of M 82 (V=9.3) when integrated over its extent => V ~ 8.2
  • Two V < 9 stars in FOV: Excesses of 1.1σ & 0.8σ at their locations (>0.7º from M 82)
  • Not due to dodgy behavior in a telescope: Signal still present when each telescope is

individually excluded

slide-12
SLIDE 12

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 12

Backup slides

slide-13
SLIDE 13

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 13

VERITAS After the move

T2 T1 T3 T4

slide-14
SLIDE 14

Niklas Karlsson - The 2009 Fermi Symposium (4 Nov 2009) 14

Improved Sensitivity